Russia is straining its relationship with Nato by holding unannounced military exercises, said the Western military alliance's deputy secretary general.

Russia had staged large drills with no advance notification with increasing frequency, said Alexander Vershbow.

He said there had been about a dozen in the past two years, and that Russia's drills are allowed by a loophole in a security agreement signed with Western countries. Mr Vershbow insists Nato wants to "develop a more stringent regime to increase transparency.